The present invention relates to apparatus and methods using such apparatus for measuring retroreflective properties of a surface. In particular, the apparatus can be useful in measuring the brightness at angles of divergence of light retroreflected from a surface and also the effect on retroreflective properties when the angle of incidence changes.
Retroreflective surfaces offer reflective capabilities not available with diffuse or mirrored surfaces with regard to reflecting incident light back toward the light source. For example, mirrored surfaces are able to reflect light back to the light source only when the light source is at a 90.degree. angle to the mirrored surfaces. Therefore, if an observer is at the light source, the mirrored surface would only appear bright when the observer is at such a 90.degree. angle to the surface.
Retroreflective surfaces have found increasing use because of their ability to reflect incident light back towards the light source. For example, retroreflective surfaces are used in constructing signs for use along roads and highways and in making easily observed "reflectors" for use on objects such as vehicles, posts, mailboxes and the like.
Currently some retroreflective surfaces are constructed by carefully arranging in a composite structure various individual optical elements. Retroreflective surfaces are also constructed by forming a surface of spherical beads over a reflective surface, such as an aluminized reflective surface. For such a retroreflective surface the incident rays are focused by the beads at about the beads' back surfaces or beyond. The light is reflected by the mirrored surface, back through the beads and refracted at the beads' front surfaces which direct the light generally back to the source.
Another type of retroreflective surface utilizes prismatic reflectors such as cube corner prisms disposed along a surface. In such prismatic reflectors the incident light is refracted upon entering the prism, thrice reflected by the back surfaces of the prism and refracted upon exiting the prism to return the light generally to its source.
It is desirable to define two terms used herein. The term "incidence angle" is the angle formed by the incident light rays to a line perpendicular to a reflective surface. The term "divergence angle" is the angle between a line formed by the source light ray (incident light ray) and the retroreflected light ray back to the observer.
The currently accepted method for measuring retroreflectivity is described in pages 11-12 and 18 of Federal Specification L-S-300C. The apparatus described therein consists of a projector, means for mounting the retroreflective sheeting and a detector that is precisely positioned near the projector. The projector has a maximum lens diameter of one inch and is capable of projecting a uniform light. The light reflected from the test surface is measured with a photoelectric receiver. The photoelectric receiver has dimensions such that no point on the perimeter is more than one-half inch from the center. The samples are mounted on a flat black test surface not less than three feet square. The sample is placed 50 feet plus or minus 2 inches from the projector lens and the receiver. To conduct the reflectivity test the angles of incidence and divergence are set and the reflectivity determined by the following equation: ##EQU1## wherein R is the reflective intensity, Er is the illumination incident upon the receiver, Es is the illumination incident upon a plane perpendicular to the incidence ray at the specimen position, d is the distance from the specimen to the projector, and A is the area of the test surface. Drawbacks of this method are that it measures the reflective properties at set angles and requires a long room in which to conduct the test.
Other instruments have been devised to facilitate measuring retroreflective properties other than using the government recommended procedure above. Such other methods avoid the necessary long, dark room (50 feet at least) needed to conduct the above test. U.S. Pat. No. 3,214,596 to Schwerdt, Jr. et al describes apparatus for detecting retroreflected light. The apparatus detects retroreflected light but does not have the capacity to measure the effect on retroreflectivity by changing the angle of incidence and divergence.
U.S. Pat. No. 3,229,564 to Metzer describes apparatus for measuring reflected light and especially specularly and diffusely reflected light. Because of the apparatus' ability to measure specularly reflected light, the apparatus would also be capable of measuring retroreflected light when the incident light is perpendicular to the mirrored surface.
U.S. Pat. No. 4,125,328 to Suga relates to apparatus for measuring reflectivity or transmissivity of a surface. In the apparatus means are provided for changing the angle of incidence and the angle of reflectance through which the intensity of the reflected light is measured. No means is provided for measuring retroreflected light or retroreflective properties of the surface.
A method and apparatus is desirable in the art which could provide direct measurement of the effects on retroreflective properties of changing the angle of incidence and angle of observation (divergence angle) of light incident to and reflected by a retroreflective surface. Apparatus which could provide a continual monitoring of retroreflective properties while such angles were changing would also be desirable in the art. Such apparatus would be especially desirable if it were compact (i.e., much less than 50 feet in length), easy to use, precise and accurate in measuring the retroreflective properties.